A number of power tools have been produced to facilitate forming a work piece into a desired shape. One such power tool is a table saw. A wide range of table saws are available for a variety of uses. Some table saws are very heavy and relatively immobile. Other table saws, sometimes referred to as job site table saws, are relatively light and portable so that a user can easily transport and position the table saw at a job site. All table saws, including cabinet table saws and job site table saws, present a safety concern because the saw blade of the table saw is typically very sharp and moving at a high rate of speed. Accordingly, severe injury such as severed digits and deep lacerations can occur almost instantaneously.
Various types of safety systems have been developed for table saws in response to the dangers inherent in an exposed blade moving at high speed. One such safety system is a blade guard. Blade guards movably enclose the saw blade, thereby providing a physical barrier that must be moved before the rotating blade is exposed. While blade guards are effective to prevent some injuries, the blade guards can be removed by a user either for convenience of using the table saw or because the blade guard is not compatible for use with a particular shaping device.
Table saw safety systems have also been developed which are intended to stop the blade when a user's hand approaches or touches the blade. Various stopping devices have been developed including braking devices which are physically inserted into the teeth of the blade. Such approaches are extremely effective. Upon actuation of this type of braking device, however, the blade is typically ruined because of the braking member. Additionally, the braking member is typically destroyed. Accordingly, the user must replace the blade and the braking member, often at considerable expense, after actuation of the safety device. Another shortcoming of this type of safety device is that the shaping device must be toothed. Moreover, if a spare blade and braking member are not on hand, a user must travel to a store to obtain replacements. Thus, while effective, this type of safety system can be expensive and inconvenient.
Some table saw safety systems, including some systems that incorporate blade braking devices, also move the blade below the surface of the table saw to prevent further contact with the user once the safety system detects an initial contact. FIGS. 1 and 2 depict a known table saw 100 that includes a safety system that moves the blade below the table saw surface in response to blade contact with the user. The table saw 100 includes a base housing 102 and a work-piece support surface 104. A riving knife or splitter 106 is positioned adjacent to a blade 108 which extends from within the base housing 102 to above the work-piece support surface 104. A blade guard (not shown) may be attached to the splitter 106. An angle indicator 110 indicates the angle of the blade 108 relative to the work-piece support surface 104. A bevel adjust turn-wheel 112 may be used to establish the angle of the blade 108 with respect to the work-piece support surface 104 by pivoting a frame 114 (FIG. 2) within the base housing 102.
A motor 116, which is powered through a switch 118 located on the base housing 102, is supported by a carriage assembly 120. The carriage assembly 120 and a stop pad 122 are supported by the frame 114. The carriage assembly 120 includes a carriage 124 to which the motor 116 is mounted and two guiderails 126/128. The position of the carriage 124 along the guiderails 126/128 is controlled by a blade height turn-wheel 130 through a gearing assembly 132 and a height adjustment rod 134. The carriage 124 fixedly supports a latch assembly 140 and supports a swing arm assembly 142.
The swing arm assembly 142 is pivotally coupled to the carriage 124 for movement between a latched position in which the blade is held above the support surface 104 and a de-latched position in which the blade is allowed to rotate into the base housing 102. As shown in FIG. 3, the swing arm assembly 142 includes a housing 144. The housing 144 encloses a power wheel (not shown) that is driven by an output shaft 152 of the motor 116. The output shaft 152 may be directly driven by the motor 116 or by a reduction gear. A belt (not shown) transfers rotational movement from the power wheel 150 to a blade wheel 156. A nut 158 is used to affix the blade 108 (not shown in FIG. 3 for purpose of clarity) to the blade wheel 156. Additionally, as shown in FIG. 3, the swing arm assembly 142 may also include a strike plate 146 and a rebound plate 148 mounted on the housing 144.
In operation, the swing arm assembly 142 is initially maintained in a latched position with the blade wheel 156 positioned sufficiently close to the work-piece support surface 104 such that the blade 108 extends above the work-piece support surface 104 as shown in FIG. 1. A user operates the bevel adjust turn wheel 112 to pivot the frame 114 with respect to the work-piece support surface 104 to establish a desired angle between the blade 108 and the work-piece support surface 104. The user further operates the blade height adjustment turn-wheel 130 to move the carriage 124 along the guiderails 126/128 to establish a desired height of the blade 108 above the work-piece support surface 104. Using the switch 118, power is then applied to the motor 116 causing the output shaft 152 and the power wheel to rotate. Rotation of the power wheel 150 causes the belt to rotate the blade wheel 156 and the blade 108 which is mounted on the blade wheel 156. A work-piece may then be shaped by moving the work-piece into contact with the blade 108.
The table saw 100 includes a sensing and control circuit (not shown) which activates an actuator, such as a solenoid or a pyrotechnic cartridge, in response to a sensed condition. Any desired sensing and control circuit may be used for this purpose. One known sensing and control circuit is described in U.S. Pat. No. 6,922,153, the entire contents of which are herein incorporated by reference. The safety detection and protection system described in the '153 patent senses an unsafe condition and provides a control signal which, in the table saw 100, is used to activate the actuator.
Upon activation of the actuator, an actuator pin is forced outwardly from the actuator. When the swing arm assembly 142 is maintained in a latched position, the strike plate 146 is aligned with the solenoid. Accordingly, as the actuator pin is forced out of the actuator, the actuator pin contacts the strike plate 146, which releases the latch assembly 140 and imparts an impact force on the swing arm assembly 142. Consequently, the swing arm assembly 142 pivots about the output shaft 152 such that the blade wheel 156 moves away from the work-piece support surface 104. Accordingly, the blade 108 is pulled by the swing arm assembly 142 in a direction away from the work-piece support surface 104.
The swing arm assembly 142 continues to pivot about the output shaft 152 until a foot 192 of the swing arm assembly 142 contacts the stop pad 122. Accordingly, further rotation of the swing arm assembly 142 is impeded by the stop pad 122. At this position, the blade 108 is completely located below the work-piece support surface 104. Therefore, an operator above the work-piece support surface 104 cannot be injured by the blade 108.
The stop pad 122 can be formed from microcellular polyurethane elastomer (MPE). MPEs form a material with numerous randomly oriented air chambers. Some of the air chambers are closed and some are linked. Additionally, the linked air chambers have varying degrees of communication between the chambers and the orientation of the linked chambers varies. Accordingly, when the MPE structure is compressed, air in the chambers is compressed. As the air is compressed, some of the air remains within various chambers, some of the air migrates between other chambers, and some of the air is expelled from the structure. One such MPE is MH 24-65, commercially available from Elastogran GmbH under the trade name CELLASTO®.
Use of an MPE or other appropriate material in the stop pad 122 stops rotation of the swing arm assembly 142 without damaging the swing arm assembly 142. However, prior to impacting the stop pad 122, the swing arm assembly 142 may be moving with sufficient force to cause the swing arm assembly to rebound off of the stop pad 122. For instance, in some table saw safety systems, a pyrotechnic device is detonated in order to generate the high-energy actuating force on the swing arm assembly 142 needed to rapidly move the blade 108 below the support surface 104 when the safety system detects contact with the user. The energy applied to rotate the swing arm assembly 142 can be partially absorbed by the material of the stop pad 122, but due to the high energy displacement of the swing arm assembly 142, some rebound is still possible. In such a circumstance, the swing arm assembly 142 will rotate about the power shaft 152 in a counterclockwise direction. Thus, the blade 108 moves toward the work-piece support surface 104.
In view of the foregoing, it would be advantageous to provide a power tool with a safety system that prevents a rotating blade from rebounding toward a user after the blade is moved away from the user in response to contact between the user and the blade. A safety system that decreases the activation time to prevent such rebounding of the blade would be further advantageous. A further advantage would be realized by a safety system that could be reset without the need for disassembly of the power tool.